You would need to look through https://github.com/brave/brave-browser/releases to find the version you’d want to download and can do it from there. Use the search box to help narrow down between types of releases.
This version of Mac lost all support back around October 2019. So it’s been a little over 3 years since nobody supported it. Really should be upgraded. Means you’re way behind on security and all. You’re open to many vulnerabilities.
Well, you could have searched for it. Quick search here found someone else who asked and had answer given to them last years. HELP! What version of brave for Mac El Capitan 10.11.6. It also was the #1 result on Google.
There’s no official way to get rid of it. I know on Windows people have found ways to change Registry. I’m not familiar with Mac on how to do it. But idea is they don’t want you using it and it’s not supported. It’s going to appear each time to make sure you or whoever is using the browser knows that the browser is vulnerable to hacking, viruses, etc and that it’s not supported, meaning many features may be missing or broken. Not to mention performance would not be as good.